Deep beneath our feet, Earth’s liquid iron outer core sloshes and churns, slowly crystallizing to form the solid inner core while simultaneously generating our planet’s magnetic field. In a recent study, Domenico Meduri, a geodynamo modeler at the University of Liverpool, focused on the past 10 million years of this erratic, roiling motion—more like river rapids than calm waters—using state-of-the-art computational facilities and refined code.
Using their own simulations, Meduri and his team successfully reproduced salient features of the paleomagnetic field preserved in volcanic rocks. Such features include not only pole reversals—where north and south swap places—but also other fundamental characteristics of the paleomagnetic field recorded by rock samples.
Two ingredients spurred Meduri’s success. He and his team found that the most commonly modeled driver of the outer core’s movements—differences in temperature—cannot explain paleomagnetic field measurements, with the composition of the swirling liquid instead playing an important role. Within the simulations, he also turned the knobs that approximate the physical characteristics of the moving molten metal, confirming that although Earth’s core mostly behaves like a dipolar bar magnet—with only two poles—it may hover between the dipolar and multipolar regimes.
The research was published in Geophysical Research Letters in January.
Stirring the Cauldron
Two phenomena drive the outer core’s turbulent movement: thermal convection and compositional convection. Thermal convection occurs because the outer core is cooling along with the rest of Earth, explained Monika Korte, a paleomagnetist from the Helmholtz Centre in Potsdam, Germany, who was not involved in the new study. As the metallic liquid loses heat, colder material sinks toward the inner core, pushing hotter liquid upward, resulting in movement within the entire outer core driven by temperature variations, she said.
The outer core is also slowly crystallizing to form the solid inner core, said Korte. However, light elements that crystallize at the boundary between the inner and outer core are too buoyant to be incorporated into Earth’s metal heart and instead rise through the fluid, stirring the cauldron from the bottom up. This, she said, is compositional convection.
To model the two drivers of convection, Meduri said his team modified where the buoyancy forces congregate. “In the chemical model, [buoyancy forces] are located close to the inner core boundary, whereas in thermal models, [buoyancy forces are distributed] throughout the whole fluid.”
None of Meduri’s solutions driven by thermal convection matched the long-term paleomagnetic field data gleaned from rocks. The only ones that worked for his team, said Korte, were “those driven by compositional convection.”
Bar Magnet Behavior
“To really have an Earth-like dynamo run—a long run that simulates thousands or millions of years of field evolution—[the run] should reflect the long-term average that we see in the data,” said Korte. The simulation should include the observation that, on average, the magnetic field tends to behave as though a bar magnet resides within Earth’s core.
But a successful simulation must also capture reversals, which “are a fundamental feature of Earth’s magnetic field,” said Meduri. Other observations from paleomagnetic data, like how variable the magnetic field intensity is and how much the geomagnetic poles wander, he said, must also be replicated.
Previous studies that could not successfully simulate paleomagnetic field data from the past 10 million years were “quite concerning,” said Meduri. If simulations of Earth’s geodynamo do not comply with paleomagnetic measurements, he said, “then what’s the point of using these models to study the magnetic field?”
Changing the buoyancy force distribution—the major difference between compositional and thermal convection—will not, by itself, create Earth-like simulations, said Meduri. Instead, Meduri and his team had to turn various knobs that change the physical properties of the modeled fluid.
These physical properties, said Korte, “are not exactly known because we cannot just go down to the Earth’s core and directly measure them.” Instead, she said, “they have to be inferred.”
For example, one of these knobs controls the vigor of the liquid outer core’s movement. Too calm? No reversals. Too turbulent? “The simulations are no longer very Earth-like,” said Meduri, and behave not as a bar magnet but as multiple unstable poles protruding in different places—a multipolar magnetic field.
What you need, Meduri said, is to turn the knob just enough to find the “sweet spot” between those two magnetic regimes, where the geomagnetic poles flip every so often while maintaining that bar magnet–like behavior on average. For these successful simulations, the magnetic field briefly exhibits multipolar behavior during the reversal before settling back down to look more like a stable bar magnet. “In this way,” he said, “we could get dipolar [bar magnet–like] models with high-enough directional and intensity variability.”
“That’s really the fundamental contribution of our work,” Meduri said. “We’ve known for at least 25 years that numerical simulations capture reversals, but do they also capture the directional and intensity variability we observe [in the rocks] on these long timescales or not?”
“Our work,” he said, “is really a bridge between purely theoretical dynamo simulations and what we observe of the Earth’s magnetic field. We were trying to match the two.”
—Alka Tripathy-Lang (@DrAlkaTrip), Science Writer
Tripathy-Lang, A. (2021), How geodynamo models churn the outer core, Eos, 102, https://doi.org/10.1029/2021EO154727. Published on 16 February 2021.
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